1. Technical Field of the Invention
The present invention relates to gas sensor controllers and, more particularly, to a gas sensor control device connected to a gas sensor such as a NOx sensor or the like for detecting a concentration of a specified component based on an output of the gas sensor.
2. Description of the Related Art
In recent years, there has been a tendency with tighter controls on exhaust emissions and fuel efficiency requirements. Thus, a need has been arisen to provide a technology related to, for instance, NOx emission reduction of a diesel engine and another technology related to failure detection on a NOx purifying device. Further, even a gasoline engine has an expanded application with a need to combust an air/fuel mixture in a lean-burn region accompanied by an increase NOx emissions. Thus, a need arises to provide a technology related to NOx emission control and another technology related to failure detection on a NOx purifying device. Because of these situations, there has been an increasing demand for a NOx sensor. Also, the NOx sensor may preferably include a sensing element of a multi-cell type employing a zirconia solid electrolyte body.
With the gas sensor involving the NOx sensor set forth above, an output error exists on a sensor current measured value due to an error factor peculiar to the sensor per se in general practice and another error factor present in a detection circuit connected to the gas sensor. Examples of the error factor of the gas sensor may include an individual difference and deterioration with age, etc. Examples of the error factor of the detection circuit may include a precision of a circuit element and a temperature characteristics, etc. Especially, with NOx sensor, a weak sensor current flows depending on a NOx concentration, resulting in a her increased risk of a drop in detecting precision due to the error factors. With an oxygen sensor (A/F sensor) arranged to detect an oxygen concentration, a sensor current is present in a mA-order. In contrast, the NOx sensor generates a sensor current in a nA-order with a difference in current level by 4 to 5 order of magnitudes.
With a prior art to address such issues, an attempt has been made to provide a detection circuit having a switch provided on a current pathway through which a sensor current flows. The switch is temporarily opened to shut off the sensor current from flowing through the detection circuit and a sensor current measured value is is acquired under such a state to allow an output error of the detection circuit to be calculated (see, for instance, Japanese Patent Application Publication No. 2005-326388).
However, with the detection circuit of such a structure having the switch provided on the electric pathway through which the sensor current flows, there is concern that an adverse affect occurs on sensor current detection caused by the switch. In case of using the switch comprised of, for instance, a semiconductor switching element, a leakage current (leak current) occurs in the switching element in the order of several tens nA. From this, there is a fear that a measuring error occurs when measuring the weak current like a phase when detecting the NOx concentration and there is room for improvement.
Meanwhile, with the gas sensor involving the NOx sensor set forth above, there is a need to detect that a function of the gas sensor is normal and examples of a detecting item include disconnection determination for the sensing element. With an automotive exhaust gas sensor, there is likelihood that disconnection determination is specified under a provision of law and regulations. Examples of technology of detecting a failure in the gas sensor may include those which detects a sensor disconnection based on an element impedance of, for instance, an A/F sensor. More particularly, in detecting impedance, a sweep variation is caused to occur in an set voltage to obtain a current change magnitude or an impedance value can be calculated during such sweep variation. Thus, failure detection is executed based on the current change magnitude or the impedance value. By conducting failure detection based on the element impedance, it becomes possible to make a query as to whether there is a normal operation or a failure operation even if the sensor output is “0”. That is, with an air/fuel ratio feedback control executed with a target on a theoretical air fuel ratio (in stoichiometric ratio), the sensor output is kept intact at nearly “0” and, even in such a case, failure such as disconnection or the like can be detected.
However, with a circuit arranged to detect a weak current like, for instance, a NOx detection signal, the weak current detection and impedance detection can be realized on a common circuit, causing a risk of deterioration in precision of NOx detection. That is, a current level resulting from impedance detection is in a mA order. On the contrary, a current level for the NOx detection signal lies in an nA-order with a difference in current level differing from each other by the 4th-order to 5th-order digits. Accordingly, it is difficult to perform both the NOx detection and the impedance detection at increased precision, causing deterioration in precision of detecting the NOx concentration.
Further, a technology has been proposed in which a detection circuit has a switch provided on a current pathway through which a NOx detected current flows (see, for instance, Japanese Patent Application Publication No. 2005-326388). With the use of such a technology, a NOx detection circuit and an impedance detection circuit can be suitably switched, thereby making it possible to extract a signal of a current level depending on needs.
However, with the structure in which the switch is provided in the current pathway through which a NOx detection current flows in the detection circuit, there is concern that the switch adversely affect on NOx current detection. That is, with the switch composed of, for instance, a semiconductor switching element, a leakage current (leak current) occurs in the witching element in the order of several tens nA. Therefore, in measuring a weak current like a phase when detecting the NOx concentration or the like, there is a risk of a measuring error and there is room for improvement.